While you are creating new branches every day, but it is a rare instance when you need to delete that branch on local and remote as well. Here is the complete process on how to delete a branch from local and push it to the desired remote.

Everything was going well with your Drupal setup on local and then GIT STRICKS BACK. You are trying to do a git checkout and moving to a different branch but now you have an error of GIT not able to unlink file and a modified file.